An organic electroluminescence device (hereinafter, electroluminescence is occasionally abbreviated as EL) is a self-emitting device based on the principle that, when an electrical field is applied, a fluorescent material emits light using energy generated by a recombination of holes injected from an anode with electrons injected from a cathode. Studies on an organic EL device formed of an organic material have been vigorously carried out since a layered organic EL device driven at low voltage was reported (for instance, Patent Literatures 1 to 8). In the layered device, tris(8-quinolinolato)aluminum is used as an emitting layer and a triphenyldiamine derivative is used as a hole transporting layer. Advantages of such a layered structure are to improve efficiency of injecting holes into the emitting layer, to improve efficiency of generating excitons by recombination due to blockage of electrons injected from the cathode, to trap the excitons generated in the emitting layer, and the like. As a device structure of such an organic EL device, a two-layered structure, a three-layered structure or the like has been well known. The two-layered structure includes a hole transporting (injecting) layer and an electron transporting/emitting layer. The three-layered structure includes a hole transporting (injecting) layer, an emitting layer and an electron transporting (injecting) layer. In such a layered device, a device structure and a preparation method have been contrived in order to improve efficiency of recombining the injected holes and electrons.
Luminescent materials of an organic EL device such as a chelate complex (e.g. a tris(8-quinolinol)aluminum complex), a coumarin complex, a tetraphenyl butadiene derivative, a distyrylarylene derivative and an oxadiazole derivative have been known. These materials, which have been reported to emit light of blue to red in visible region, are expected to be applied to a color-display device.
Moreover, in addition to a fluorescent material, application of a phosphorescent material has been recently proposed for the emitting layer of the organic EL device. Thus, in the emitting layer of the organic EL device, a singlet state and a triplet state of excited states of an organic phosphorescent material are used to achieve a high luminous efficiency. When electrons and holes are recombined in the organic EL device, it is presumed that a singlet exciton and a triplet exciton are produced at a rate of 1:3 due to difference in spin multiplicity. Accordingly, luminous efficiency of the device using a phosphorescent material can reach three to four times as much as that of the device only using a fluorescent material.
In forming the emitting layer, a doping method, according to which the above luminescent material (dopant) is doped to a host material, has been known.
The emitting layer formed by the doping method can efficiently generate excitons from electric charges injected into the host material. With the exciton energy generated by the excitons being transferred to the dopant, the dopant can emit light with high efficiency.
Recently, in order to upgrade an organic EL device, a further study on the doping method has been made to seek favorable host materials.
Such a host material is disclosed in, for instance, Patent Literatures 1 to 6. Patent Literature 1 discloses a compound containing a carbazole skeleton and a benzimidazole ring in the same molecule, and data of the compound as a blue fluorescent material host. Patent Literature 2 discloses a compound containing a carbazole skeleton and a 1,2,4-triazole ring in the same molecule, and data of the compound as a blue to blue-green fluorescent material host. Patent Literature 3 discloses a compound containing a carbazole skeleton and an imidazopyridine ring in the same molecule, and data of the compound as a blue fluorescent material host. Patent Literature 4 discloses a compound containing a carbazole skeleton, a fluorene skeleton and an imidazopyridine ring (a nitrogen-containing aromatic ring) in the same molecule, and data of the compound as a blue fluorescent material host. Patent Literature 5 discloses a compound containing a carbazole skeleton and a nitrogen-containing aromatic ring in the same molecule, and data of the compound as a host material used together with a blue to blue-green phosphorescent material. Patent Literature 6 discloses a compound containing a carbazole skeleton, a fluorene skeleton and a phenanthroline ring (a nitrogen-containing aromatic ring) in the same molecule, and data of the compound as a green phosphorescent material host.
Patent Literatures 7 and 8 disclose compounds having the same skeletons as those of the compounds Patent Literatures 1 to 6. Patent Literature 7 discloses a compound containing a carbazole skeleton and a pyridine ring in the same molecule, and data of the compound serving as a hole transporting material. Patent Literature 8 discloses a compound containing a fluorene skeleton and a pyridine ring (a nitrogen-containing aromatic ring) in the same molecule, and application of the compound to the emitting layer.